pH
PH is only a measurement of the free Hydrogen ions in the water and is very, very dynamic. The pH can change overnight...In particular, the pH is prone to fall, and "crashes" are quick, and fatal. Interestingly, many people say they checked the pH last week, and are surprised to find that the pH can change overnight.
In simple terms, the pH is supported in range by the carbonate activity of the water (measured in Total Alkalinity) and when the carbonates are exhausted, or overloaded, the pH plummets to 5.5 and kills filter bacteria and fish. In other situations, the pH can glide downward very slowly over months and thus your resident fish can survive a low pH but the new fish you keep buying perish quickly as they enter the water with a very acid pH.
Check the pH regularly, and also, to get a grip on your 'safety net', measure the water's carbonate activity as Total Alkalinity as well.
Buy carbonates for the pond in a sensibly priced pH Buffer.
Buy a pH test kit
Buy a Total Alkalinity test kit.